1. Field of the Invention
The present invention relates to a scale, particularly for a position measuring device. The scale includes a base member which is fastened by means of an adhesive layer to a graduation carrier with a graduation.
2. Description of the Related Art
A scale of the above-described type is used particularly in a position measuring device for measuring the relative positions of two objects, for example, two machine components of a processing machine.
German patent 25 05 587 describes a length measuring device in which a graduation carrier is fastened to a housing as the base member without direct contact by means of an elastic adhesive layer, so that measuring inaccuracies are avoided when the base member and the graduation carrier have different thermal coefficients of expansion.
German Utility Model 89 11 313 describes a scale in which a graduation carrier of glass is fastened on a base member of steel by means of a thin inelastic adhesive layer of a hard adhesive, so that the behavior of the base member of steel in the case of temperature changes is imparted through the thin layer of hard adhesive to the graduation carrier of glass and, consequently, the behavior of the scale in the case of temperature changes corresponds approximately to the behavior of the machine part of steel to which it is fastened. The surface of the base member and/or the surface of the graduation carrier which come into contact with the adhesive have recesses for receiving the excessive hard adhesive during the gluing procedure, so that a layer of hard adhesive is obtained which is as thin as possible and transmits as much force as possible.
German Auslegeschrift 26 43 304 describes a length measuring device in which a graduation carrier rests on the edges of a rectangular groove of a housing as the base member, wherein the groove extends in the measuring direction. The graduation carrier is connected to the base member in the middle of the graduation carrier by means of an inelastic adhesive in the groove and on both sides by means of a highly elastic adhesive in the groove, so that a thermal expansion is obtained which is independent of the base member and is uniform from the middle, whereby the middle of the graduation carrier maintains its position unchanged relative to the base member.
The adhesion of the adhesive layer to the surfaces of the base member and/or of the graduation carrier is more or less satisfactory depending on the surface properties and the manner in which the surfaces have been cleaned, so that the adhesive layer may be at least partially separated from the surfaces of the base member and/or of the graduation carrier and, consequently, the planeness of the graduation surface of the graduation carrier is impaired and the scanning graduation surface of the scanning unit is changed, which may lead to measuring inaccuracies and to problems in operation.